Steel for surface hardening for machine structural use and part for machine structural use

ABSTRACT

The present invention is steel for surface hardening for machine structural use which contains, by mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%, Mn: 0.35 to less than 1.5%, and Al: 0.01 to 0.5%, is restricted to B: less than 0.0003%, S: 0.0001 to 0.021%, N: 0.003 to 0.0055%, P: 0.0001 to 0.03%, and O: 0.0001 to 0.0050%, has a ratio Mn/S of Mn and S satisfying 70 to 30,000, has a balance of Fe and unavoidable impurities, and, when nitrided, then induction hardened, has a surface hardenability of a Vicker&#39;s hardness when tempered at 300° C. of 650 or more.

This application is a national stage application of InternationalApplication Nos. PCT/JP2010/050742, filed Jan. 15, 2010, which claimspriority to Japanese Application No. 2009-007756, filed on Jan. 16,2009, which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to steel for surface hardening for machinestructural use and a part for machine structural use. More particularly,it relates to parts which have high surface fatigue strength which areused for automobiles and other power transmission parts, in particular,gears, continuously variable transmissions, bearings, constant velocityjoints, hubs, and other parts for machine structural use.

BACKGROUND ART

Parts for machine structural use, for example, gears of automatictransmissions and sheaves of continuously variable transmissions,bearings, constant velocity joints, hubs, and other power transmissionparts are required to have a high surface fatigue strength. In the past,for the above parts, JIS SCr420, SCM420, and other case hardened steelswith C of around 0.2% have generally been used for the material, while ahardened layer of a martensite structure with C of around 0.8% has beenformed on the surface of the part by carburized quenching so as to raisethe surface fatigue strength in use.

However, carburized quenching is treatment which takes a long time of 5to 10 hours, in some cases more than 10 hours, along with the austenitetransformation at the high temperature of around 950° C., so heattreatment deformation (quenching strain) due to the crystal graincoarsening unavoidably becomes greater. For this reason, parts for whicha high dimensional precision has been demanded have had to be ground,honed, and otherwise finished after carburized quenching.

In recent years, there has been rising demand for reducing the noise ofautomobile engines etc., so surface hardening with less heat straincompared with carburized quenching, such as induction hardening and softnitriding, have come under the spotlight.

Induction hardening heats a steel material in a short time. Since onlythe necessary part of the surface layer is transformed to austenite andhardened, there is little hardening strain and it is possible to obtaina surface hardened part with a high dimensional precision.

However, to obtain a hardness equivalent to that of a carburizedquenched material by only induction hardening, it is necessary to add0.8% or more of C to the steel material. The hardness of the inside ofthe material, which has no relation to improvement of the surfacefatigue strength, also rises and remarkable deterioration of themachineability occurs. Therefore, it is not possible to just increasethe amount of C in the steel material without proper consideration, sothere is a limit to improving the surface fatigue strength by justinduction hardening.

Soft nitriding is a surface hardening method which causes the diffusionand permeation of mainly nitrogen and carbon simultaneously at the steelmaterial surface in the temperature region below the transformationpoint of about 500 to 600° C. so as to form a hardened layer and improvethe wear resistance, seizing resistance, fatigue resistance, etc.

At the steel material surface, the diffused nitrogen forms nitrides inthe steel, forms a compound layer comprised of mainly Fe₃N, Fe₄N, andother Fe nitrides at the surfacemost layer of a general steel material,and forms a nitrided layer in which N is diffused inside from thesurfacemost layer of the steel material.

Soft nitriding can be performed at a low temperature. Compared with thecase of carburized quenching, a short treatment time of about 2 to 4hours is enough, so this is often applied to steel parts where lowstrain is required.

However, with just soft nitriding, the hardened layer depth is small, soapplication to a gear etc. of a transmission at which a high surfacepressure is applied is difficult.

Recently, as a method for compensating for the defects in inductionhardening and soft nitriding and obtaining better mechanical properties,in particular improving the surface fatigue strength, performing softnitriding, then induction hardening is being experimented with.

PLT 1 proposes a method of production which combines gas soft nitridingand induction hardening so as to make up for their individual defectsand improve the softening resistance and obtain superior mechanicalproperties, particularly high surface fatigue strength.

The method of production of PLT 1 treats a steel material by gas softnitriding to form a compound layer, then treats this by inductionhardening to break up and diffuse into the steel the nitrogen compoundsin the compound layer which is formed by the gas soft nitriding so as toform a hardened nitrided layer.

Note that, in the following explanation, the layer which is comprised ofFe₃N, Fe₄N, and other Fe nitrides which are formed at the surfacemostlayer of the steel material by soft nitriding will be referred to as the“compound layer”, while the nitrided layer which is formed by diffusionof N inside of the steel material from the surfacemost layer, whenformed without induction hardening, will be referred to as a “nitridedlayer” and, when formed with induction hardening, will be referred to asa “hardened nitrided layer” so as to differentiate them.

The steel material which is produced by the method of production of PLT1 is high in surface hardness, but is low in concentration of N in thehardened nitrided layer, so the hardness of the steel material at thetime of a high temperature is low and it is not possible to obtain asufficient softening resistance at the surface of gears etc. whichbecome a high temperature during operation. As a result, it is notpossible to obtain a high surface fatigue strength.

PLT 2 proposes a method of production which combines soft nitriding andinduction hardening to obtain a part for machine structural use superiorin mechanical properties. In the method of production of PLT 2, elementswith a high affinity with N are added to the steel material so as tocause the nitrides in the steel material to break up and be diffused.

However, with the method of production of PLT 2, the amounts of additionof the elements for breaking up and diffusing the nitrides in the steelmaterial are not sufficient, so it is necessary to heat the steelmaterial to 900° C. to 1200° C., an extremely high temperature, byinduction heating and make the N form a solid solution in the steel. Forthis reason, a thick oxide layer is formed at the steel materialsurface. Due to that oxide layer, the steel material is unavoidablyremarkably degraded in mechanical properties.

Further, when converting the compound layer which is obtained by softnitriding to a hardened nitrided layer by induction hardening, in themethod of production of PLT 2, no thought is given to a method ofincreasing the thickness of the hardened nitrided layer.

Therefore, the part for machine structural use which is obtained by themethod of production of PLT 2 is not sufficient in the thickness of thehardened nitrided layer, so does not have a surface fatigue strengthgood enough for use at a high surface pressure.

PLT 3 proposes the art of combining nitriding and induction hardening toobtain a part for machine structural use which has superior mechanicalproperties. The part for machine structural use of PLT 3 is obtained bynitriding a steel material at a 600° C. or more high temperature to forma compound layer, then performing induction hardening to form a hardenednitrided layer.

However, the nitriding in PLT 3 is performed at a 600° C. or more hightemperature, so the compound layer which is formed is thin and theconcentration of N in the compound layer is also low. Therefore, even ifnitriding, then induction hardening, the nitrogen compounds in thecompound layer which is formed by the nitriding are decomposed and theamount of N which diffuses to the inside of the steel material is small.

That is, with nitriding performed at a 600° C. or more high temperature,even if it is possible to form a compound layer, then perform inductionhardening to form a hardened nitrided layer, the thickness of thehardened nitrided layer is insufficient, a sufficient softeningresistance cannot be obtained, and as a result a part for machinestructural use which has a good surface fatigue strength cannot beobtained.

PLT 4 proposes a method of production of a machine structure partcomprising performing soft nitriding under conditions giving a nitridedlayer depth of 150 μm or more, then performing induction hardening underconditions where the nitrided layer is transformed to austenite so as tothereby form a hardened nitrided layer.

However, the part for machine structural use which is produced by themethod of production which is proposed in PLT 4 has a thickness of thehardened nitrided layer of 0.3 mm even at the maximum. The surfacefatigue strength is not sufficient.

PLT 5 proposes a part for machine structural use which is obtained byheat treating a hot worked steel material for graphite precipitation,then cold working it and finally nitriding it.

However, the part for machine structural use of PLT 5 uses theprecipitated graphite to improve the machineability. The precipitatedgraphite at the steel material surface causes the surface fatiguestrength to drop.

Therefore, even if treating the part for machine structural use of PLT 5by induction hardening to form a hardened nitrided layer, it isdifficult to use the part for machine structural use of PLT 5 as a gearor other power transmission part of a transmission where a high surfacepressure is applied to the surface of the part for machine structuraluse.

Further, in general, gears and other power transmission parts areobtained by forging, then machining the materials to finish them to theshapes of the parts, then surface hardening them to obtain the completedparts. The proposals in the above PLTs 1 to 5 are arts aimed at raisingthe strength of the operating surfaces by treating medium carbon steelcontaining alloy elements for surface hardening.

Therefore, since the machineability is not considered, the unnecessaryrise in hardness at the inside of the steel material causes a drop inthe productivity at the time of machining and therefore themanufacturing costs unavoidably rise.

Accordingly, it is desired to improve the surface fatigue strength of asteel material while keeping down a rise in hardness inside the steelmaterial and preventing a drop in machineability.

CITATIONS LIST Patent Literature

-   PLT 1: Japanese Patent Publication (A) No. 6-172961-   PLT 2: Japanese Patent Publication (A) No. 7-90363-   PLT 3: Japanese Patent Publication (A) No. 2007-77411-   PLT 4: Japanese Patent Publication (A) No. 7-90364-   PLT 5: Japanese Patent Publication (A) No. 2008-169485

SUMMARY OF INVENTION Technical Problems

The present invention, in view of the above situation, has as its objectthe provision of steel for surface hardening for machine structural usewhich can be used for parts which have a good dimensional precisionwhich cannot be obtained by carburized quenching, which is improved inthe surface hardness, inside hardness, and softening resistance whichare insufficient with soft nitriding alone or induction hardening alone,which have a high surface fatigue strength, and which are subjected to ahigh surface pressure not only at ordinary temperature of course, butalso a 300° C. or so high temperature, and a steel part for machinestructural use which uses that steel for surface hardening for machinestructural use.

Solution to Problem

To improve the surface fatigue strength of a steel part, it is necessaryto improve the surface hardness, increase the surface hardened layerdepth, and improve the softening resistance for maintaining hightemperature strength at an operating surface which becomes a hightemperature (around 300° C.).

Further, to prevent a drop in the productivity, it is also desirable toprevent a drop in the machineability of a material along withimprovement of the surface fatigue strength and to form a lubricatingfilm at the steel material surface for preventing seizing or sticking ofthe operating surfaces with each other.

To deal with these problems, the inventors engaged in various studies onthe surface hardenability of steel materials involving combining softnitriding and induction hardening so as to improve the surface fatiguestrength of the steel materials and also studied machineability andlubricating films. As a result, they obtained the following findings:

a) To increase the softening resistance, it is effective to make thehardened nitrided layer which is formed on the steel material surfacethicker and make the concentration of N in the hardened nitrided layerhigher.

Soft nitriding forms a compound layer on the surfacemost layer of asteel material and forms a nitrided layer at the inside from thatsurfacemost layer. However, the compound layer which is formed by softnitriding is very thin. The nitrided layer which is formed inside fromthe compound layer also does not have a thickness which is sufficientfor increasing the softening resistance. The concentration of N insideof the nitrided layer is also not sufficient.

Therefore, the invention performs soft nitriding, then inductionhardening so as to break down the compound layer which is formed by thesoft nitriding (layer mainly comprised of Fe₃N, Fe₄N, and other Fenitrides) by the induction heating, make a sufficient amount of Ndiffuse inside the steel, and form a hardened nitrided layer.

The thus obtained hardened nitrided layer gives the steel material asufficient softening resistance and results in a Vicker's hardness of650 or more when tempered at 300° C. Further, the steel material whichhas such a hardened nitrided layer has a good surface fatigue strengthand can be used not only at ordinary temperature of course, but also atthe time of a 300° C. or so high temperature.

FIG. 1 is a view showing an example of comparison of the cross-sectionalhardness distributions of steel materials from the surfaces to the coredirections for a steel material as soft nitrided and a steel materialwhich is soft nitrided, then induction hardened. In FIG. 1, referencenumeral 1 shows the cross-sectional hardness distribution of a steelmaterial as soft nitrided, while reference numeral 2 shows the hardnessdistribution of the steel material which is soft nitrided, theninduction hardened.

As shown in FIG. 1, the steel material 1 as soft nitrided is formed witha compound layer at its surfacemost layer. This exhibits an extremelyhigh hardness, but the thickness of the compound layer is small. On theother hand, the steel material 2 which is soft nitrided, then inductionhardened has Fe nitrides which are present in the compound layer of thesurfacemost layer decomposed by induction heating and has the N derivedfrom the decomposed Fe nitrides diffused inside of the steel material.As a result, while the hardness of the surfacemost layer of the steelmaterial 2 falls somewhat, a hardened nitrided layer which hassufficient hardness is formed thickly inside from the surfacemost layer.That is, the compound layer of the surfacemost layer which is formed bythe soft nitriding functions as a source of N for formation of thehardened nitrided layer.

Note that, the surface layer of the steel material 2 after the inductionhardening is a martensite structure, while the core part is aferrite-pearlite structure.

By making the thickness of the compound layer which is decomposed by theinduction heating 10 μm or more, a high N concentration hardenednitrided layer is deeply obtained. The compound layer which is formed bythe soft nitriding becomes brittle depending on the soft nitridingconditions and sometimes degrades the mechanical properties, so thecompound layer is generally made thin.

In the present invention, the compound layer is deliberately madethicker. That is, by making the compound layer 10 μm or more, it ispossible to make the hardened nitrided layer which is formed byinduction hardening a high N concentration martensite structure. Thesteel material is remarkably increased in softening resistance.

b) To form a thick compound layer by soft nitriding, it is effective toreduce the S interfering with the bonding of N with the steel. If the Sin the steel material forms a solid solution alone, the S will easilyconcentrate at the steel material surface and will obstruct entry of Ninto the steel material surface. To prevent this, a certain amount ormore of Mn is added and the S is immobilized in the steel in the stateof MnS and thereby rendered harmless. The effect of such renditionremarkably appears by making Mn/S≧70. As a result, it is possible toform a compound layer of a 10 μm or more thickness.c) To prevent the machineability from being degraded even if raising thesurface fatigue strength, it is preferable to not raise the hardness ofthe inside of the steel material more than necessary. Furthermore, it ispreferable to add elements which raise the surface fatigue strengthwhile improving the machineability.

To prevent the hardness of the inside of the steel material from risingmore than necessary, it is effective to not excessively add Mn, N, andother alloy elements.

Further, it is also possible to add Al and B compositely as elementswhich improve the surface fatigue strength while improving themachineability. B combines with the N in the steel to remain in thesteel as BN and thereby improves the machineability.

B forms BN during the cooling in a forging process. In a forgingprocess, the cooling speed of the steel material is usually slow, soeven if forming BN, the hardness is not raised and the machineability isnot lowered.

The BN which is formed in the forging process is decomposed by theinduction heating at the time of the induction hardening and becomessolid solution B. By the rapid cooling at the time of hardening, thiscauses the surface layer hardness of the steel material to greatly riseand also contributes to the improvement of the surface fatigue strength.

Al is an element which remains in the steel in a solid solution state tothereby remarkably improve the machineability. Al has almost no effectin raising the hardness of the steel material. Further, at the time ofsoft nitriding, Al has the effect that it forms a compound with N andraises the concentration of N near the surface layer. It is also anelement effective for improving the surface fatigue strength.

Further, by adding Al and B compositely, the B forms BN which iseffective for improvement of the machineability. Further, this BN isdecomposed by induction hardening into B and N. It is therefore possibleto obtain a hardened nitrided layer with a high concentration of N. Theresultant B improves the hardenability of the steel material, so a highsurface fatigue strength can be obtained.

Furthermore, by forming BN and thereby consuming the N in the steelmaterial, it is possible to keep Al from forming a compound with N andobtain more solid solution Al. The machineability is also improved.

d) To prevent seizing or sticking of the operating surfaces, it iseffective to provide an oil reservoir so that a film of a lubricant iscontinuously formed on the surface of a steel part. The steel materialof the present invention has a hardened nitrided layer which is obtainedby using soft nitriding to form a compound layer, then breaking up theFe nitrides in the compound layer by induction heating and transformingthe steel material to austenite for hardening.

FIG. 2 give views showing the structures of a hardened nitrided layerwhich is observed under an optical microscope and scan type electronmicroscope. FIG. 2A shows the structure which is observed under anoptical microscope, while FIG. 2B shows the structure which is obtainedunder the scan type electron microscope.

As shown in FIG. 2, the hardened nitrided layer 10 has a large number ofholes 20 which are formed by decomposition of the nitrogen compounds inthe compound layer and forms a hard porous layer 30. This porous layer30 functions as an oil reservoir. It improves the lubrication effect andfurther improves the wear resistance and durability of the steelmaterial.

Note that, by controlling the soft nitriding conditions and inductionheated conditions, it is possible to make the size of the hole 20 0.1 to1 μm in an equivalent circle diameter, the density of presence of theholes 20 10,000 holes/mm² or more, and the range of presence of theholes 20 a depth of 5 μm or more from the surface of the steel part.These effectively function as an oil reservoir.

The present invention was completed based on the above findings and hasas its gist the following:

(1) Steel for surface hardening for machine structural use whichcontains, by mass %,

C: 0.3 to 0.6%,

Si: 0.02 to 2.0%,

Mn: 0.35 to less than 1.5%, and

Al: 0.01 to 0.5%,

is restricted to

B: less than 0.0003%,

S: 0.0001 to 0.021%,

N: 0.003 to 0.0055%,

P: 0.0001 to 0.03%, and

O: 0.0001 to 0.0050%,

has a ratio Mn/S of Mn and S satisfying 70 to 30,000,

has a balance of Fe and unavoidable impurities, and

has a surface hardenability of a Vicker's hardness of 650 or more afternitriding, induction hardening, and tempering at 300° C.

(2) Steel for surface hardening for machine structural use whichcontains, by mass %,

C: 0.3 to 0.6%,

Si: 0.02 to 2.0%,

Mn: 0.35 to less than 1.5%,

Al: 0.01 to 0.5%, and

B: 0.0003 to 0.005%,

is restricted to

S: 0.0001 to 0.021%,

N: 0.003 to 0.0055%,

P: 0.0001 to 0.03%, and

O: 0.0001 to 0.0050%,

has a ratio Mn/S of Mn and S satisfying 70 to 30,000,

has a balance of Fe and unavoidable impurities, and

has a surface hardenability of a Vicker's hardness of 650 or more afternitriding, induction hardening, and tempering at 300° C.

(3) Steel for surface hardening for machine structural use as set forthin (1) or (2), characterized in that the steel further contains, by mass%, one or more of

W: 0.0025 to 0.5%,

Cr: 0.2 to 2.0%,

Mo: 0.05 to 1.0%,

V: 0.05 to 1.0%,

Nb: 0.005 to 0.3%,

Ti: 0.005 to 0.2%,

Ni: 0.05 to 2.0%, and

Cu: 0.01 to 2.0%.

(4) Steel for surface hardening for machine structural use as set forthin (1) or (2), characterized in that the steel further contains, by mass%, one or more of

Ca: 0.0005 to 0.01%,

Mg: 0.0005 to 0.01%,

Zr: 0.0005 to 0.05%, and

Te: 0.0005 to 0.1%.

(5) Steel for surface hardening for machine structural use whichcontains, by mass %,

C, 0.3 to 0.6%,

Si: 0.02 to 2.0%,

Mn: 0.35 to less than 1.5%, and

Al: 0.01 to 0.5%,

contains one or more of

W: 0.0025 to 0.5%,

Cr: 0.2 to 2.0%,

Mo: 0.05 to 1.0%,

V: 0.05 to 1.0%,

Nb: 0.005 to 0.3%,

Ti: 0.005 to 0.2%,

Ni: 0.05 to 2.0%, and

Cu: 0.01 to 2.0%,

contains one or more of

Ca: 0.0005 to 0.01%,

Mg: 0.0005 to 0.01%,

Zr: 0.0005 to 0.05%, and

Te: 0.0005 to 0.1%,

is restricted to

B: less than 0.0003%,

S: 0.0001 to 0.021%,

N: 0.003 to 0.0055%,

P: 0.0001 to 0.03%, and

O: 0.0001 to 0.0050%,

has a ratio Mn/S of Mn and S satisfying 70 to 30,000,

has a balance of Fe and unavoidable impurities, and

has a surface hardenability of a Vicker's hardness of 650 or more afternitriding, induction hardening, and tempering at 300° C.

(6) Steel for surface hardening for machine structural use whichcontains, by mass %,

C: 0.3 to 0.6%,

Si: 0.02 to 2.0%,

Mn: 0.35 to less than 1.5%,

Al: 0.01 to 0.5%, and

B: 0.0003 to 0.005%,

contains one or more of

W: 0.0025 to 0.5%,

Cr: 0.2 to 2.0%,

Mo: 0.05 to 1.0%,

V: 0.05 to 1.0%,

Nb: 0.005 to 0.3%,

Ti: 0.005 to 0.2%,

Ni: 0.05 to 2.0%, and

Cu: 0.01 to 2.0%,

contains one or more of

Ca: 0.0005 to 0.01%,

Mg: 0.0005 to 0.01%,

Zr: 0.0005 to 0.05%, and

Te: 0.0005 to 0.1%,

is restricted to

S: 0.0001 to 0.021%,

N: 0.003 to 0.0055%,

P: 0.0001 to 0.03%, and

O: 0.0001 to 0.0050%,

has a ratio Mn/S of Mn and S satisfying 70 to 30,000,

has a balance of Fe and unavoidable impurities, and

has a surface hardenability of a Vicker's hardness of 650 or more afternitriding, induction hardening, and tempering at 300° C.

(7) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(1) or (2) which, when nitrided, then induction hardened and tempered at300° C., has a Vicker's hardness of 650 or more down to a depth of 0.2mm from the surface of the steel for surface hardening for machinestructural use.(8) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(1) or (2) which, when nitrided, then induction hardened to form ahardened nitrided layer and tempered at 300° C., has a Vicker's hardnessof 650 or more and has holes 0.1 to 1 μm in an equivalent circlediameter down to a depth of at least 5 μm from the surface of thehardened nitrided layer present on a scale of 10,000 holes/mm² or more.(9) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(1) or (2) which, when nitrided, then induction hardened to form ahardened nitrided layer and tempered at 300° C., has a Vicker's hardnessof 650 or more down to a depth of 0.2 mm from the surface of the steelfor surface hardening for machine structural use and has holes of 0.1 to1 μm in an equivalent circle diameter down to a depth of at least 5 μmfrom the surface of the hardened nitrided layer present on a scale of10,000 holes/mm² or more.(10) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(3) which, when nitrided, then induction hardened and tempered at 300°C., has a Vicker's hardness of 650 or more down to a depth of 0.2 mmfrom the surface of the steel for surface hardening for machinestructural use.(11) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(3) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more and has holes 0.1 to 1 μm in an equivalent circle diameter downto a depth of at least 5 μm from the surface of the hardened nitridedlayer present on a scale of 10,000 holes/mm² or more.(12) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(3) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more down to a depth of 0.2 mm from the surface of the steel forsurface hardening for machine structural use and has holes of 0.1 to 1μm in an equivalent circle diameter down to a depth of at least 5 μmfrom the surface of the hardened nitrided layer present on a scale of10,000 holes/mm² or more.(13) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(4) which, when nitrided, then induction hardened and tempered at 300°C., has a Vicker's hardness of 650 or more down to a depth of 0.2 mmfrom the surface of the steel for surface hardening for machinestructural use.(14) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(4) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more and has holes of a 0.1 to 1 μm in an equivalent circle diameterdown to a depth of at least 5 μm from the surface of the hardenednitrided layer present on a scale of 10,000 holes/mm² or more.(15) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(4) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more down to a depth of 0.2 mm from the surface of the steel forsurface hardening for machine structural use and has holes 0.1 to 1 μmin an equivalent circle diameter down to a depth of at least 5 μm fromthe surface of the hardened nitrided layer present on a scale of 10,000holes/mm² or more.(16) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(5) which, when nitrided, then induction hardened and tempered at 300°C., has a Vicker's hardness of 650 or more down to a depth of 0.2 mmfrom the surface of the steel for surface hardening for machinestructural use.(17) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(5) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more and has holes 0.1 to 1 μm in an equivalent circle diameter downto a depth of at least 5 μm from the surface of the hardened nitridedlayer present on a scale of 10,000 holes/mm² or more.(18) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(5) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more down to a depth of 0.2 mm from the surface of the steel forsurface hardening for machine structural use and has holes 0.1 to 1 μmin an equivalent circle diameter down to a depth of at least 5 μm fromthe surface of the hardened nitrided layer present on a scale of 10,000holes/mm² or more.(19) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(6) which, when nitrided, then induction hardened and tempered at 300°C., has a Vicker's hardness of 650 or more down to a depth of 0.2 mmfrom the surface of the steel for surface hardening for machinestructural use.(20) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(6) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more and has holes 0.1 to 1 μm in an equivalent circle diameter downto a depth of at least 5 μm from the surface of the hardened nitridedlayer present on a scale of 10,000 holes/mm² or more.(21) A steel part for machine structural use characterized by comprisingsteel for surface hardening for machine structural use as set forth in(6) which, when nitrided, then induction hardened to form a hardenednitrided layer and tempered at 300° C., has a Vicker's hardness of 650or more down to a depth of 0.2 mm from the surface of the steel forsurface hardening for machine structural use and has holes 0.1 to 1 μmin an equivalent circle diameter down to a depth of at least 5 μm fromthe surface of the hardened nitrided layer present on a scale of 10,000holes/mm² or more.

Advantageous Effects of Invention

The steel for surface hardening for machine structural use of thepresent invention, by soft nitriding, then induction hardening, isremarkably increased in hardness at the surface of the steel materialand is increased in softening resistance to thereby give a high surfacefatigue strength.

The parts for machine structural use of the present invention can beused for power transmission parts of automobiles etc. for which a highsurface fatigue strength is demanded not only at ordinary temperature ofcourse, but also under usage conditions resulting in a high temperatureof around 300° C., for example, gears, continuously variabletransmissions, bearings, constant velocity joints, hub's, etc. Theygreatly contribute to the higher output and lower cost of automobilesetc.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing an example of comparison of the cross-sectionalhardness distributions of steel materials from the surfaces to the coredirections for a steel material as soft nitrided and a steel materialwhich is soft nitrided, then induction hardened.

FIG. 2 gives views showing the structure of a hardened nitrided layerobserved by an optical microscope and scan type electron microscope.FIG. 2A shows the structure which is observed by an optical microscope,while FIG. 2B shows the structure which is observed by a scan typeelectron microscope.

FIG. 3 is a view showing the relationship between the Mn/S at the timeof soft nitriding and the compound thickness.

FIG. 4 is a view showing the relationship between the concentration of Nat a depth of 0.2 mm from the surface of the steel after inductionhardening and the Vicker's hardness (Hv) at the time of tempering at300° C.

DESCRIPTION OF EMBODIMENTS

The present invention treats a steel material, to which suitable amountsof Si, Mn, and Al have been added, by soft nitriding, then inductionhardening to form a deep hardened nitrided layer with a highconcentration of N and improve the surface hardness and softeningresistance to thereby obtain a high surface fatigue strength not only atordinary temperature of course, but also at a 300° C. or so hightemperature.

First, the reasons for limitation of the necessary added elements inpresent invention will be explained. Note that, the % of the chemicalcomposition show the mass %.

C, 0.3 to 0.6%

C is an important element for obtaining the strength of the steel. Inparticular, it is an element which is necessary for reducing the ferritefraction of the micro structure before induction hardening, improvingthe hardenability at the time of induction hardening, and increasing thehardened layer depth. If the amount of addition of C is less than 0.3%,the ferrite fraction is high and the hardening at the time of inductionhardening becomes insufficient, so the lower limit of the amount ofaddition of C was made 0.3%. On the other hand, if the amount ofaddition of C is excessive, the machineability and forgeability at thetime of part fabrication are remarkably impaired and, furthermore, thepossibility of cracks occurring at the time of induction hardeningbecomes greater, so the upper limit of the amount of addition of C wasmade 0.6%.

Si: 0.02 to 2.0%

Si has the effect of increasing the softening resistance of the hardenedlayer so as to improve the surface fatigue strength. To obtain thateffect, the amount of addition of Si has to be at least 0.02%. On theother hand, if the amount of addition of Si is over 2.0%, thedecarburization at the time of forging becomes remarkable, so 2.0% wasmade the upper limit.

Mn: 0.35 to less than 1.5%

Mn is an element effective for improving the hardenability andincreasing the softening resistance to improve the surface fatiguestrength. Furthermore, it has the effects of immobilizing the S in thesteel as MnS so as to prevent S from concentrating at the steel materialsurface and N from invading the steel and of promoting the formation ofa thick compound layer by soft nitriding. To immobilize the S in thesteel as MnS to render it harmless, it is necessary to make Mn/S≧70.Further, it lowers the ferrite fraction of the structure beforeinduction hardening and improves the hardenability at the time ofinduction hardening. To obtain that effect, Mn has to be added in 0.35%or more. If adding a suitable amount of Mn, the hardenability at thetime of induction hardening is improved, so the hardness of the steelmaterial after hardening rises and the surface fatigue strength isimproved. However, if adding Mn in 1.5% or more, the hardness of thematerial rises more than necessary, the machineability of the materialbefore soft nitriding is remarkably degraded, and the productivity isdegraded. For this reason, the amount of addition of Mn is made lessthan 1.5%.

Mn/S: 70 to 30000

As explained above, to prevent the concentration of S at the steelmaterial surface, it is necessary to add Mn to S by a certain ratio ormore and render the S harmless by MnS. If the ratio Mn/S of the amountsof addition of Mn and S is 70 or more, the effect of rendering the Sharmless is remarkably improved. However, if Mn/S is less than 70, Sconcentrates at the steel material surface and formation of a compoundlayer at the time of soft nitriding is inhibited, so Mn/S was made 70 ormore.

FIG. 3 is a view showing the relationship between the Mn/S at the timeof soft nitriding under the conditions explained later and the compoundthickness. As clear from FIG. 3, by making Mn/S 70 or more, after thesoft nitriding, a 10 μm or more thickness compound layer is obtained. Onthe other hand, even if Mn/S exceeds 30000, the effect of rendering theS harmless becomes saturated, so the upper limit of Mn/S was made 30000.

Al: 0.01 to 0.5%

Al is an element which precipitates and diffuses in the steel as Alnitrides and thereby effectively acts to increase the grain fineness ofthe austenite structure at the time of induction hardening and,furthermore, improves the hardenability and increases the hardened layerdepth. Further, it is an element which is effective for improvement ofthe machineability. Therefore, the amount of addition of Al has to be0.01% or more. Further, it is an element which has the effect of bondingwith N at the time of soft nitriding and raising the concentration of Nnear the surface layer of the steel material and is also effective forimproving the surface fatigue strength. Therefore, the amount ofaddition of Al has to be made 0.01% or more. On the other hand, if theamount of addition of Al exceeds 0.5%, the precipitates (Al nitrides)will coarsen and cause the steel to become brittle, so the upper limitwas made 0.5%.

B: less than 0.0003%

B is unavoidably included in steel. Even so, by restricting the contentof B to less than 0.0003%, it does not detract from the advantageouseffect of the present invention.

B: 0.0003 to 0.005%

If B is added into the steel, the N and B in the steel bond to form BNin the steel, but at the time of induction heating, the BN is decomposedand releases the B. This greatly improves the hardenability and improvesthe surface fatigue strength. To obtain that effect, the amount ofaddition of B has to be made 0.0003% or more: On the other hand, even ifthe amount of addition of B is over 0.005%, that effect becomessaturated. Furthermore, it becomes a cause of cracking at the time ofrolling and forging, so 0.005% was made the upper limit. Note that, BNis formed during heat treatment with a slow cooling speed and duringcooling in cold working with a normally slow cooling speed. Therefore,at the time of machining performed after cold working and before softnitriding and induction hardening, BN improves the machineability. Aftermachining, induction hardening causes the BN to break up resulting in ahardened nitrided layer. That hardened nitrided layer improves thesurface fatigue strength. Therefore, this is perfect for production of apart for machine structural use where high surface fatigue strength isdemanded.

S: 0.0001 to 0.021%

S has the effect of improving the machineability. However, S is a softnitriding inhibiting element which concentrates at the steel materialsurface to thereby obstruct the entry of N to the steel material at thetime of soft nitriding. If the amount of addition of S exceeds 0.021%,the entry of N into the steel material is remarkably inhibited and,furthermore, the forgeability is remarkably degraded. Therefore, toimprove the machineability, when including S, it is necessary to makethe content 0.021% or less. On the other hand, the industrial lowerlimit of the amount of addition of S was made 0.0001%. Note that, asexplained above, to immobilize the S in the steel as MnS and eliminateits inhibitory effect on soft nitriding, the lower limit of Mn/S is made70. On the other hand, even if Mn/S exceeds 30,000, the effect ofeliminating the inhibitory effect on soft nitriding becomes saturated,so the upper limit of Mn/S has to be made 30,000.

N: 0.003 to 0.0055%

N forms various types of nitrides and is effective for increasing thegrain fineness of the austenite structure at the time of inductionhardening. To obtain that effect, it is necessary to make the amount ofaddition of N 0.003% or more. On the other hand, if excessive N isadded, the hardness rises. Furthermore, the N and Al bond to form AlN,whereby the amount of solid solution Al effective for improvement of themachineability is reduced, so the machineability is degraded. Further,the excessively added N causes the ductility in the high temperatureregion to drop. Furthermore, it forms coarse AlN or coarse BN, so makesthe material remarkably brittle resulting in cracks at the time ofrolling and forging. Therefore, the amount of addition of N has to belimited to 0.0055% or less.

P: 0.0001 to 0.03%

P segregates at the grain boundaries and causes the toughness to drop,so is preferably reduced as much as possible. It has to be limited to0.03% or less. The lower limit of the amount of addition of P is madethe industrial limit of 0.0001%.

O: 0.0001 to 0.0050%

O is present in the steel as Al₂O₃, SiO₂, and other oxide-basedinclusions, but if the amount of O is great, such oxides become large insize. Such enlarged oxides form starting points for breakage of powertransmission parts, so the content of O has to be limited to 0.0050% orless. The smaller the content of O, the better, so 0.0020% or less ismore preferable. In the case of power transmission parts aiming at longservice lifetime, 0.0015% or less is further preferable. Note that thelower limit of the content of O is made the industrial limit of 0.0001%.

Next, the reasons for restriction of the optionally added elements willbe explained.

Elements for Strengthening Steel Material

W: 0.0025 to 0.5%

W is an element which improves the hardenability and thereby improvesthe surface fatigue strength. However, due to the addition of W, thehardness of the steel material rises and the machineabilitydeteriorates, so there is a limit to the addition of W. To improve thehardenability so as to achieve a great improvement in the surfacefatigue strength, the amount of addition of W is preferably made 0.0025%or more. More preferably, it is made 0.03% or more. On the other hand,if the amount of addition of W exceeds 0.5%, that effect becomessaturated and the economy is impaired, so 0.5% was made the upper limit.

Cr: 0.2 to 2.0%

Cr has the effect, by addition, of improving the nitridingcharacteristics and the softening resistance of the hardened layer andof improving the surface pressure fatigue strength. To obtain thateffect, the amount of addition of Cr is preferably made 0.2% or more. Onthe other hand, if the amount of addition of Cr exceeds 2.0%, themachineability deteriorates, so the upper limit of the amount ofaddition of Cr is preferably made 2.0%.

Mo: 0.05 to 1.0%

Mo, by addition, has the effect of improving the softening resistance ofthe hardened layer and improving the surface fatigue strength plus theeffect of strengthening and toughening the hardened layer to improve thebending fatigue strength. To obtain these effects, the amount ofaddition of Mo is preferably made 0.05% or more. On the other hand, evenif the amount of addition of Mo exceeds 1.0%, these effects aresaturated and economy is impaired, so the upper limit of the amount ofaddition of Mo is preferably made 1.0%.

V: 0.05 to 1.0%

V, by addition, precipitates and diffuses as nitrides in the steel andis effective for increasing the grain fineness of the austenitestructure at the time of induction hardening. To obtain that effect, theamount of addition of V has to be made 0.05% or more. On the other hand,even if the amount of addition of V exceeds 1.0%, that effect becomessaturated and economy is impaired, so the upper limit of the amount ofaddition of V is preferably made 1.0%.

Nb: 0.005 to 0.3%

Nb, by addition, precipitates and diffuses as nitrides in the steel andis effective for increasing the grain fineness of the austenitestructure at the time of induction hardening. To obtain that effect, theamount of addition of Nb is preferably 0.005% or more. On the otherhand, even if the amount of addition of Nb exceeds 0.3%, that effectbecomes saturated and economy is impaired, so the upper limit of theamount of addition of Nb is preferably made 0.3%.

Ti: 0.005 to 0.2%

Ti, by addition, precipitates and diffuses as nitrides in the steel andis effective for increasing the grain fineness of the austenitestructure at the time of induction hardening. To obtain that effect, theamount of addition of Ti is preferably 0.0005% or more. On the otherhand, if the amount of addition of Ti exceeds 0.2%, the precipitates (Tinitrides) coarsen and cause the steel to become brittle, so the upperlimit of the amount of addition of Ti is preferably made 0.2%.

Ni: 0.05 to 2.0%

Ni, by addition, has the effect of further improving the toughness. Toobtain that effect, it is preferable to make the amount of addition ofNi 0.05% or more. On the other hand, if the amount of addition of Niexceeds 2.0%, the machineability deteriorates, so the upper limit of theamount of addition of Ni is preferably made 2.0%.

Cu: 0.01 to 2.0%

Cu is effective for strengthening the ferrite and improving thehardenability and corrosion resistance. If the amount of addition of Cuis less than 0.01%, that effect cannot be observed, so the amount ofaddition of Cu is preferably made 0.01% or more. On the other hand, evenif the amount of addition of Cu exceeds 2.0%, the effect of improvementof the mechanical properties due to improvement of the hardenabilitybecomes saturated, so the upper limit of the amount of addition of Cu ispreferably made 2.0%. Note that, if adding Cu, the hot rollability islowered and defects are easily caused at the time of rolling, so Cu ispreferably added simultaneously with Ni.

Elements for Improving Bending Strength

When improvement of the bending fatigue strength of the part for machinestructural use is sought, one or more of the following contents of Ca,Mg, Zr, and Te may be added in the following ranges.

Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to 0.05%, Te:0.0005 to 0.1%

Ca, Mg, Zr, and Te are all elements which suppress the flattening of MnSpresent in the steel, improve the bending fatigue strength more, andmake bending fatigue fracture of the gears and fatigue fracture of thespline bottoms of shaft parts more difficult. The effect of suppressionof flattening of MnS is obtained by adding Ca in 0.0005% or more, Mg in0.0005% or more, Zr in 0.0005% or more, and Te in 0.0005% or more.Therefore, it is preferable to make Ca: 0.0005%, Mg: 0.0005%, Zr:0.0005%, and Te: 0.0005% the lower limits of the amounts of addition. Onthe other hand, even if adding Ca in more than 0.01%, Mg in more than0.01%, Zr in more than 0.05%, and Te in more than 0.1%, the effect ofsuppression of flattening of MnS becomes saturated and the economy isimpaired. Therefore, it is preferable to make Ca: 0.01%, Mg: 0.01%, Zr:0.05%, and Te: 0.1% the upper limits of the amounts of addition.

Further, in addition to the elements prescribed above, it is possible toinclude Pb, Bi, Sn, Zn, Rem, and Sb in ranges not impairing theadvantageous effects of the present invention.

Next, the thickness and hardness of the hardened nitrided layer at thesurface layer of the steel part will be explained.

The steel part of the present invention is a steel part which is treatedby soft nitriding, then induction hardening to form a hardened nitridedlayer. The surface hardenability of steel after forming the hardenednitrided layer has to be a Vicker's hardness of 650 or more whentempered at 300° C. If the Vicker's hardness is less than 650 whentempered at 300° C., the softening resistance is not sufficientlyincreased and, as a result, cracks occur and surface fatigue fractureoccurs at the operating surfaces (steel part surfaces) which become highin temperature (around 300° C.).

Further, the range of a Vicker's hardness of 650 or more when temperedat 300° C. is preferably down to a depth of 0.2 mm from the surface ofthe steel. If the range of a Vicker's hardness of 650 or more whentempered at 300° C. is shallower than 0.2 mm from the surface of thesteel, the steel part cannot withstand the surface pressure which isapplied. In particular, it cannot withstand the surface pressure whichis applied at a 300° C. or so high temperature. The steel part thereforefractures by fatigue.

Further, the thickness of the hardened nitrided layer is preferably 0.4mm or more. If the thickness of the hardened nitrided layer is less than0.4 mm, when the steel part is subjected to a high surface pressure,surface cracks are formed. Before these surface cracks form startingpoints for fracture, internal fracture occurs due to spalling andtherefore the fatigue life of the steel part becomes shorter.

In the actual steel part, whether the steel part is one which is softnitrided, then induction hardened can be judged from the distribution ofmicrostructures which are observed by an optical microscope after Nitalcorrosion of a sample taken from the steel part and the distribution ofhardness from the surface to the core.

To form a hardened nitrided layer and obtain a high surface fatiguestrength, it is necessary to perform soft nitriding to form a compoundlayer (a layer mainly comprised of Fe₃N, Fe₄N, and other Fe nitrides) atthe surfacemost layer of the steel. The Fe nitrides in the compoundlayer are decomposed by the induction heating performed after the softnitriding whereby a sufficient amount of N is made to diffuse into thesteel and a hardened nitrided layer is formed. That is, it is necessaryto perform soft nitriding to form a compound layer serving as the sourceof supply of N for forming a hardened nitrided layer. Further, thehardened nitrided layer which is obtained by soft nitriding, theninduction hardening exhibits a high N concentration.

To make a sufficient amount of N diffuse in the steel and thickly form ahardened nitrided layer which is hard and can improve the softeningresistance, in particular the hardened softening resistance, it ispreferable to make the thickness of the compound layer after softnitriding 10 μm or more.

Further, the soft nitriding temperature is preferably made 500 to 600°C. in range. If the soft nitriding temperature is over 600° C., it isnot possible to make the compound layer 10 μm or more. Furthermore, theconcentration of N in the compound layer also becomes lower. Further, ifthe soft nitriding temperature exceeds 600° C., heat deformation, grainboundary oxidation, etc. of the steel material occur. On the other hand,if the soft nitriding temperature is less than 500° C., the penetrationof N into the steel material remarkably falls, so the lower limit of thesoft nitriding temperature is preferably made 500° C.

If lengthening the soft nitriding time, the compound layer and nitridedlayer depth become larger, but that effect becomes saturated in about 3hours, so the soft nitriding time is preferably 1 to 3 hours.

The cooling after the soft nitriding may be performed by any of themethods of air cooling, N₂ gas cooling, oil cooling, etc.

Further, as the soft nitriding, either of gas soft nitriding or saltbath soft nitriding may be applied.

Note that, so long as a method by which the steel material surface issupplied with nitrogen and the surfacemost layer of the steel materialis formed with a 10 μm or more compound layer, not only soft nitriding,but also nitriding (meaning a surface hardening method treating thesurface by only NH₃ for a long period of time and industriallydifferentiated from soft nitriding which uses a mixed atmosphere of NH₃and CO₂ (in some cases, a mixed atmosphere further having N₂ mixed in)to enable treatment in a short period of 1 to 3 hours) may be applied.

In the present invention, soft nitriding is more preferable thannitriding in the point that the surfacemost layer of the steel can beformed with a 10 μm or more compound layer in a short time, but theinvention is not limited to soft nitriding.

Further, to break up the compound layer which is formed by softnitriding on the surfacemost layer of the steel material and cause N todiffuse from that surfacemost layer to the inside of the steel materialand thereby obtain a high surface hardenability of a Vicker's hardnessof 650 or more when tempered at 300° C., it is necessary, after the softnitriding, to treat the steel material by induction hardening whichheats the material by induction heating to convert it to austenite forhardening and form a hardened nitrided layer.

The heating method which performing the induction hardening has toconsider the breakup of the compound layer which is formed by the softnitriding. The induction heated temperature is made the austenitetransformation point to less than 900° C. Further, the holding time ismade 0.05 to 5 seconds. Note that, the “holding time” means the time forholding the temperature of the steel material in the range of theaustenite transformation point to less than 900° C. from the point oftime where the temperature of the induction heated steel materialreaches the austenite transformation point.

FIG. 4 is a view showing the relationship between the concentration of Nat a depth of 0.2 mm from the surface of the steel after inductionhardening and the Vicker's hardness (Hv) at the time of tempering at300° C.

As clear from FIG. 4, when the Vicker's hardness after 300° C. temperingat a depth of 0.2 mm from the surface of the steel is 650 or more, theconcentration of N at a depth of 0.2 mm from the surface of the steelwas 0.5% or more.

Further, if the temperature of the induction heating is 900° C. or more,N will unnecessarily diffuse to the inside, the concentration of N at adepth of 0.2 mm from the surface will not become 0.5% or more, theVicker's hardness when tempered at 300° C. will become less than 650,and, as a result, the surface fatigue strength will not be able to beimproved. Further, if the temperature of the induction heating is 900°C. or more, the increase in the oxide layer at the steel materialsurface will cause degradation of the mechanical properties.

On the other hand, if the temperature of the induction heating is lessthan the austenite transformation point, the steel material will nottransform to martensite, so it is not possible to obtain a high surfacehardness.

If the holding time is less than 0.05 second, the breakup of thecompound layer and the diffusion of the N produced by the decompositionof the compound layer will become insufficient. On the other hand, ifthe holding time is over 5 seconds, N will unnecessarily diffuse to theinside, the concentration of N at a depth of 0.2 mm from the surfacewill not become 0.5% or more, the Vicker's hardness when tempered at300° C. will become less than 650, and, as a result, the surface fatiguestrength will not be able to be improved.

The frequency when performing induction heating is preferably, for asmall part, around 400 kHz and, for a large part, around 5 kHz.

The coolant which is used for the hardening may be water, a polymerquenchant, or other water-based coolant with a large cooling ability.

After induction hardening, it is preferable to perform low temperaturetempering of around 150° C. in accordance with general carburizedquenched parts so as to secure the toughness of the part.

Next, the surface layer structure of the steel material and steel partof the present invention will be explained.

The steel material and steel part of the present invention are softnitrided, then induction hardened, so have 10,000 holes/mm² or moreholes of 0.1 to 1 μm in an equivalent circle diameter down to a depth of5 μm or more from the surface.

For example, in a member like a gear in which the surface fatigue due torolling becomes a cause of fracture, the lubrication of the operatingsurface is important. If not sufficiently lubricated, the parts willcontact each other resulting in seizing or sticking and surface fatiguefracture will occur. To form a sufficient lubricating film, it iseffective to provide an oil reservoir at the operating surface so that afilm of lubricant is continuously formed.

The steel material and steel part of the present invention have, at thesurfacemost layer of the steel material, a hardened nitrided layer whichis obtained by using soft nitriding to form a compound layer mainlycomprised of Fe₃N, Fe₄N, and other Fe nitrides, breaking up these Fenitrides by induction heating, and transforming the steel to austenitefor hardening. This hardened nitrided layer is formed by the Fe nitridesin the compound layer being decomposed and the N which is releaseddiffusing to the inside of the steel material. In this process offormation, the locations where the Fe nitrides were present in thecompound layer form a large number of dispersed holes. The hardenednitrided layer therefore becomes a hard porous layer. Further, theselarge number of dispersed holes function as an oil reservoir whereby thelubrication effect is improved and the steel material is furtherimproved in wear resistance and durability.

The holes are of a size of 0.1 to 1 μm in an equivalent circle diameterand a density of 10,000 holes/mm² or more. Further, these holes functionas an oil reservoir if present down to a depth of 5 μm or more from thesurface. Such holes are obtained by controlling the conditions of thesoft nitriding and induction heating.

Even a compound layer as soft nitrided has a small number of holes, sothese have the function as an oil reservoir, but the compound layer assoft nitrided is extremely brittle and cannot withstand a large surfacepressure, so the steel material as soft nitrided invites surface fatiguefracture.

If the size of the holes is a circle equivalent diameter of over 1 μm,the surface smoothness of the steel part will deteriorate and form thestarting points for pitching and other surface fatigue fracture and thesurface fatigue strength will be lowered. On the other hand, if the maindimension of the holes is an equivalent circle diameter of less than 0.1μm, a sufficient function as an oil reservoir cannot be obtained.

If the density of the holes is less than 10,000 holes/mm², the holeswill not effectively function as an oil reservoir.

Further, gears and other sliding members are generally used until beingworn down about 5 μm from the surface of the members, so the holes arepreferably present down to a depth of 5 μm or more from the surface ofthe steel part.

The size and density of the holes depend on the soft nitriding andinduction heating conditions. To obtain the size and density of holeswhich effectively function as an oil reservoir, preferably the softnitriding temperature is made 580° C. to less than 600° C., theinduction heating temperature is made 880° C. to less than 900° C., andthe holding time is made 1 to 4 seconds. Note that, these conditions ofcourse satisfy the conditions for obtaining a hardened nitride layerprovided at a steel material and steel part having a high surfacefatigue strength.

Further, the surface layer after hardening was made a martensitestructure, while the core was left as a ferrite-pearlite structure. Thisis because by making only the surface layer transform to martensite, thesurface layer is given compressive residual stress and the surfacefatigue strength is improved. If transforming even the core part tomartensite, the compressive residual stress of the surface layer ends upbeing reduced and the surface fatigue strength falls.

Note that, what was explained above was just an illustration of anembodiment of the present invention. Various changes may be made withinthe scope of description of the claims.

Examples

Next, the present invention will be explained further by examples, butthe conditions of the examples are only illustrations of the conditionswhich are employed for confirming the workability and advantageouseffects of the present invention. The present invention is not limitedto these illustrations of the conditions. The present invention canemploy various conditions so long as not departing from the gist of thepresent invention and achieving the object of the present invention.

Each of the steel materials having the chemical compositions shown inTables 1 to 2 and Tables 4 to 5 was forged and annealed, then fabricatedinto roller pitching test pieces for evaluation of the surface fatiguestrength, that is, a small roller test piece having a cylindrical partwith a diameter of 26 mm and a width of 28 mm and a large roller testpiece with a diameter of 130 mm and a width of 18 mm.

The small roller test piece and large roller test piece were softnitrided (nitrided for 2 hours at temperatures shown in Table 3 andTable 6, then cooled by N₂ gas, nitriding gas composition: N₂ (0.45Nm³/h)+NH₃ (0.5 Nm³/h)+CO₂ (0.05 Nm³/h)), then induction hardened(frequency 100 kHz). The coolant used at the time of induction hardeningwas tapwater or a polymer quenchant. After that, the test pieces weretempered at 150° C. for 60 minutes and used for fatigue tests.

The fabricated small roller test piece and large roller test piece wereused for a standard surface fatigue test, that is, a roller pitchingfatigue test.

The roller pitching fatigue test was performed by having the smallroller test piece pushed against by various hertz stresses (surfacepressures) by the large roller test piece and setting the slip rate at−40% (peripheral speed of large roller test piece being 40% larger thansmall roller test piece at contacting parts of the small roller testpiece and the large roller test piece). Note that, the rotationaldirection and the contacting parts of the small roller test piece andthe large roller test piece are made the same. Further, the temperatureof the gear oil which was fed to the contacting parts of the smallroller test piece and large roller test piece was made 90° C.

The cutoff number of the test was made the 10 million cycles (10⁷cycles) showing the fatigue limit of general steel, while the maximumhertz stress at which 10 million cycles was reached at the small rollertest piece without the occurrence of pitching was made the fatigue limitof the small roller test piece. The occurrence of pitching was detectedby a vibration meter attached to the tester. After detection ofvibration, the rotations of both the small roller test piece and thelarge roller test piece were made to stop and the occurrence of pitchingand rotational speed were confirmed.

Further, for evaluation of the temper softening resistance, acylindrical hardness measurement test piece of a diameter of 26 mm and alength of 100 mm was fabricated. The hardness measurement test piece wassoft nitrided and induction hardened under the same conditions for thesmall roller test piece and large roller test piece. After that, thiswas tempered at 300° C. for 60 minutes, was cut cross-sectionally, thenwas measured for the hardness distribution from the surface to the coreof the hardness measurement test piece by a Vicker's hardness meter.Note that, the surface layer of the hardness measurement test pieceafter induction hardening was a martensite structure, while the corepart was left as a ferrite-pearlite structure. Further, together withthis, the concentration of N at a depth of 0.2 mm from the surface ofthe hardness measurement test piece was measured by EPMA.

Further, the density of the holes which have an equivalent circlediameter of 0.1 to 1 μm was found by cutting a hardness measurement testpiece, which was soft nitrided and induction hardened under the sameconditions as the small roller test piece and large roller test piece,at a cross-section perpendicular to rolling, burying this in resin,polishing it to a mirror finish, then image processing the surfacemostlayer part. The image processing was performed at 3000× for 40 fields of50 μm² each. The number of holes found by image processing was convertedto the number of holes per mm².

Furthermore, for evaluation of the machineability, a cylindrical testpiece of a diameter of 45 mm and a length of 100 mm was fabricated. Themachineability is evaluated in the state of the material before softnitriding and induction hardening, so the test piece for evaluation ofthe machineability was used as is for forging and annealing. Themachineability was evaluated by a deep hole drilling test using MQL(minimum quantity lubrication) by an NC machining center used forproduction of gears, crankshafts, and other auto parts. The number ofholes drilled until drill breakage when drilling under the conditionsshown in Table 7 was measured. However, when reaching 1000 or moreholes, the machineability was judged to be good and the test was cutoff.

Table 3 and Table 6 show the results. As shown in Table 3, in each ofthe invention examples of Examples 1 to 40, the surface fatigue strength(maximum hertz stress) at 10 million cycles (10⁷ cycles) in a rollerpitching fatigue test is a high value of 3700 MPa or more. It is clearthat each has a superior surface fatigue strength. It was confirmed thatit is possible to obtain good results compared with the comparativeexamples of Examples 41 to 62 shown in Table 6. Due to such superiorsurface fatigue strength, the steel material and steel part of thepresent invention and steel can be used for members upon which a highsurface pressure is applied both at ordinary temperature of course andalso at a high temperature of around 300° C.

For example, each of the invention examples of Examples 1 to 8 is asteel to which Si, Mn, and Al are added in suitable amounts. Due to theformation of a compound layer of a thickness of 10 μm or more by softnitriding of less than 600° C. and the 0.08 to 4.9 seconds of inductionhardening after that at the austenite transformation point to 900° C.,it could be confirmed that a Vicker's hardness of 650 or more after 300°C. tempering is obtained and, as a result, a superior surface fatiguestrength is obtained. Further, in each of the invention examples ofExamples 1 to 8, the thickness of the hardened nitrided layer is 0.4 mmor more. It could be confirmed that the concentration of N down to aposition 0.2 mm from the surface is high. Further, from the number ofholes drilled being 1000 or more, it could be confirmed that themachineability after forging (before soft nitriding and inductionhardening) was also superior.

Further, even in Examples 9 to 24 where optional elements were added,the surface fatigue strength (maximum hertz stress) at the 10,000million cycle (10⁷ cycle) in a roller pitching fatigue strength is ahigh value of 0.3700 MPa or more. It could be confirmed that a goodsurface fatigue strength is obtained.

Further, in each of the invention examples of Examples 25 to 40, thereare 10,000 holes/mm² or more holes of an equivalent circle diameter of0.1 to 1 μm present down to a depth of 5 μm from the surface of thehardened nitrided layer, but the surface fatigue strength (maximum hertzstress) at the 10,000 million cycle (10⁷ cycle) in a roller pitchingfatigue strength is a high value of 3700 MPa or more. It could beconfirmed that a good surface fatigue strength is obtained.

As opposed to this, in each of the comparative examples of Examples 41to 62, where steels with chemical compositions outside the range of thepresent invention are soft nitrided, then induction hardened, thefatigue test lifetime was the value of the surface fatigue strength(maximum hertz stress) at a 10 million cycle (10⁷ cycle) lifetime ofless than 3700 MPa. Compared with the invention examples, it could beconfirmed that the surface fatigue strength was inferior.

In the comparative example of Example 42, the amount of addition of Bwas over the upper limit of the present invention, while in thecomparative example of Example 43, the amount of addition of N was overthe upper limit, so each steel was remarkably brittle, cracks occurredduring forging, and the steel could not be evaluated.

In each of the comparative examples of Examples 44 and 48, the Mn/S islow, concentration of S at the steel material surface cannot beprevented, and, for this reason, the thickness of the compound layerafter soft nitriding is thin, the Vicker's hardness after 300° C.tempering is less than 650, and, as a result, the surface fatiguestrength (maximum hertz stress) is low. Further, the thickness of thehardened nitrided layer of the steel part after induction hardening is athin one of less than 0.4 mm, while the concentration of N down to adepth of 0.2 mm from the surface is also low.

The comparative example of Example 49 has a low Mn/S, so has a lowthickness of the compound layer, and has a Vicker's hardness after 300°C. tempering of less than 650, so, as a result, it was confirmed, thesurface fatigue strength (maximum hertz stress) is low.

Each of the comparative examples of Examples 51 to 54 has a chemicalcomposition within the range of the present invention, but has aVicker's hardness of less than 650 after 300° C. tempering and, as aresult, it was confirmed, has a maximum hertz stress of less than 3000MPa. This is because, in each of Examples 51 to 54, while the compoundlayer after soft nitriding has a sufficient thickness, in Example 51,the induction heating temperature is too high, so the N unnecessarilydiffuses to the inside of the steel material, while the thickness of thehardened nitrided layer is a sufficient 0.65 mm, the Vicker's hardnessafter 300° C. tempering is 506, or less than 650, and, furthermore, anoxide layer is formed at the steel material surface, so the surfacefatigue strength (maximum hertz stress) is lowered. Further, theconcentration of N at a depth of 0.2 mm from the surface of the steelmaterial is also a low 0.09%. In Example 53, the holding time (inductionheating time) is too long, so the N unnecessarily diffuses to the insideof the steel material, while the thickness of the hardened nitridedlayer is a sufficient 0.70 mm, the Vicker's hardness after 300° C.tempering is 540, or less than 650, and, as a result, the surfacefatigue strength (maximum hertz stress) is low. Further, theconcentration of N at a depth of 0.2 mm from the surface of the steelmaterial is also a low 0.20%.

Each of the comparative examples of Examples 55 to 5 has a Vicker'shardness of less than 650 after 300° C. tempering and, as a result, itwas confirmed the surface fatigue strength (maximum hertz stress) isless than 3000 MPa. This is because the compound layer after softnitriding is thin, so the thickness of the hardened nitrided layer isalso thin. For example, in Example 56, the chemical composition iswithin the range of the present invention, but the soft nitridingtemperature is too low, so the compound layer is thin, the Vicker'shardness after 300° C. tempering is less than 650, and, as a result, thesurface fatigue strength (maximum hertz stress) is low. Further, thethickness of the hardened nitrided layer is a thin 0.16 mm.

The comparative example of Example 57 has an amount of addition of Mnsmaller than the range of the present invention, has an amount ofaddition of S greater than the range of the present invention, has anMn/S smaller than the range of the present invention, and has too high asoft nitriding temperature, so the compound layer is thin, the Vicker'shardness after 300° C. tempering is 387, or less than 650, and, as aresult, it was confirmed, the surface fatigue strength (maximum hertzstress) is low. Further, the thickness of the hardened nitrided layer isa thin 0.15 mm, while the concentration of N at a depth of 0.2 mm fromthe surface of the steel material is a low 0.08%.

Each of the comparative examples of Example 58 to 60 has a Vicker'shardness of less than 650 after 300° C. tempering and, as a result, hasa low surface fatigue strength (maximum hertz stress). This is becauseeach of Examples 58 to 60 has a thin compound layer and unsuitableinduction heating conditions, so has a thin hardened nitrided layer. Forexample, Example 58 has an amount of addition of Mn lower than the rangeof the present invention and has an Mn/S smaller than the range of thepresent invention, so the compound layer is thin. Further, the inductionheating temperature is high, so the Vicker's hardness after 300° C.tempering becomes 507, or less than 650, and, as a result, the surfacefatigue strength (maximum hertz stress) is low. Further, the thicknessof the hardened nitrided layer is a thin 0.14 mm, while theconcentration of N at a depth of 0.2 mm from the surface of the steelmaterial is a low 0.07%.

The comparative example of Example 62 has an Mn/S of 15 or lower thanthe range of the present invention and has a high soft nitridingtemperature, so the compound layer is thin, and has a high inductionheating temperature, so has a Vicker's hardness of less than 650 after300° C. tempering, as a result of which the surface fatigue strength(maximum hertz stress) is an extremely low 2600 MPa. Further, thethickness of the hardened nitrided layer is a thin 0.23 mm, while theconcentration of N at a depth of 0.2 mm from the surface of the steelmaterial is a low 0.16%.

From the above, it could be confirmed that the invention examples, whichare comprised of steel to which Si, Mn, and Al are added in suitableamounts and where Mn/S is made a suitable, range, which are softnitriding, then are induction hardened, and which have surfacehardenability of a Vicker's hardness of 650 or more when tempered at300° C., exhibit a superior surface fatigue strength of a maximum hertzstress of 3700 MPa or more.

TABLE 1 Chemical composition (mass %) Example Class C Si Mn P S Al O N B 1 Inv. ex. 0.32 0.61 0.66 0.014 0.006 0.120 0.002 0.0034 0.0004  1aInv. ex. 0.55 0.19 0.81 0.020 0.008 0.030 0.001 0.0039 0.0001  1b Inv.ex. 0.53 0.19 0.79 0.025 0.010 0.022 0.002 0.0044 0.0002  2 Inv. ex.0.37 0.59 1.15 0.022 0.008 0.035 0.001 0.0034 0.0005  3 Inv. ex. 0.490.45 1.16 0.009 0.009 0.069 0.004 0.0042 0.0010  4 Inv. ex. 0.51 1.201.40 0.025 0.016 0.153 0.002 0.0040 0.0006  5 Inv. ex. 0.54 0.25 0.800.019 0.007 0.126 0.003 0.0031 0.0015  6 Inv. ex. 0.55 0.85 0.96 0.0120.010 0.159 0.001 0.0038 0.0023  7 Inv. ex. 0.55 1.32 1.23 0.021 0.0140.080 0.002 0.0041 0.0004  8 Inv. ex. 0.59 0.85 0.72 0.021 0.006 0.0990.004 0.0049 0.0038  9 Inv. ex. 0.54 1.30 0.85 0.017 0.009 0.025 0.0020.0030 0.0004 10 Inv. ex. 0.46 0.85 0.79 0.023 0.009 0.021 0.003 0.00430.0009 11 Inv. ex. 0.44 0.69 1.07 0.015 0.010 0.192 0.001 0.0044 0.000812 Inv. ex. 0.41 1.41 1.44 0.014 0.011 0.102 0.001 0.0040 0.0017 13 Inv.ex. 0.51 0.79 0.85 0.022 0.012 0.110 0.001 0.0036 0.0005 14 Inv. ex.0.55 0.34 1.12 0.015 0.008 0.099 0.001 0.0044 0.0009 15 Inv. ex. 0.450.91 0.67 0.020 0.009 0.089 0.001 0.0042 0.0006 16 Inv. ex. 0.45 0.751.29 0.021 0.018 0.021 0.003 0.0037 0.0005 17 Inv. ex. 0.35 0.40 0.500.020 0.005 0.089 0.004 0.0049 0.0005 18 Inv. ex. 0.46 0.54 0.71 0.0200.007 0.021 0.002 0.0041 0.0005 19 Inv. ex. 0.45 1.41 0.53 0.010 0.0070.087 0.005 0.0050 0.0021 20 Inv. ex. 0.51 0.29 0.85 0.023 0.007 0.0450.004 0.0050 0.0005 21 Inv. ex. 0.50 1.67 0.56 0.021 0.004 0.089 0.0040.0042 0.0004 22 Inv. ex. 0.49 0.55 0.83 0.013 0.010 0.310 0.004 0.00500.0029 23 Inv. ex. 0.48 1.00 1.20 0.024 0.010 0.110 0.004 0.0040 0.000824 Inv. ex. 0.45 0.89 1.32 0.011 0.011 0.109 0.002 0.0038 0.0010 25 Inv.ex. 0.40 0.25 0.55 0.010 0.005 0.082 0.001 0.0041 0.0006 26 Inv. ex.0.45 1.43 0.69 0.011 0.009 0.096 0.001 0.0042 0.0015 27 Inv. ex. 0.480.59 1.00 0.020 0.009 0.085 0.002 0.0038 0.0029 28 Inv. ex. 0.51 1.000.85 0.018 0.007 0.101 0.003 0.0031 0.0006 29 Inv. ex. 0.55 1.56 0.750.013 0.008 0.123 0.002 0.0042 0.0005 30 Inv. ex. 0.56 1.90 0.84 0.0200.006 0.030 0.003 0.0040 0.0005 31 Inv. ex. 0.43 0.25 0.43 0.017 0.0030.193 0.003 0.0051 0.0033 32 Inv. ex. 0.50 0.95 0.75 0.020 0.005 0.1020.003 0.0043 0.0006 33 Inv. ex. 0.48 0.72 1.02 0.013 0.007 0.110 0.0030.0035 0.0010 34 Inv. ex. 0.41 0.81 0.75 0.013 0.009 0.099 0.002 0.00400.0004 35 Inv. ex. 0.45 0.31 0.83 0.020 0.011 0.123 0.001 0.0033 0.000436 Inv. ex. 0.53 0.51 1.01 0.011 0.013 0.025 0.002 0.0041 0.0009 37 Inv.ex. 0.57 0.46 1.07 0.015 0.015 0.243 0.002 0.0051 0.0022 38 Inv. ex.0.50 0.66 1.13 0.010 0.011 0.034 0.002 0.0046 0.0007 39 Inv. ex. 0.451.01 0.83 0.018 0.010 0.025 0.003 0.0046 0.0010 40 Inv. ex. 0.42 0.260.58 0.016 0.006 0.027 0.002 0.0036 0.0004

TABLE 2 (Continuation of Table 1) Chemical composition (mass %) Ex.Class Cr Mo W V Nb Ti Ni Cu Ca Mg Zr Te Mn/S  1 Inv. ex. 116  1a Inv.ex. 101  1b Inv. ex. 79  2 Inv. ex. 142  3 Inv. ex. 126  4 Inv. ex. 90 5 Inv. ex. 108  6 Inv. ex. 94  7 Inv. ex. 91  8 Inv. ex. 126  9 Inv.ex. 0.75 93 10 Inv. ex. 0.28 90 11 Inv. ex. 0.19 0.23 112 12 Inv. ex.0.13 135 13 Inv. ex. 0.51 0.12 74 14 Inv. ex. 1.13 0.09 145 15 Inv. ex.1.30 0.10 71 16 Inv. ex. 0.79 0.31 72 17 Inv. ex. 0.43 0.14 0.22 0.09 9918 Inv. ex. 1.84 0.12 0.14 0.44 0.10 105 19 Inv. ex. 0.02 0.11 77 20Inv. ex. 0.30 0.10 0.0011 0.0011 121 21 Inv. ex. 0.38 0.05 0.48 0.110.0044 148 22 Inv. ex. 1.63 0.69 0.25 0.12 0.14 0.009 82 23 Inv. ex.0.79 0.35 0.11 0.13 0.0009 0.0008 122 24 Inv. ex. 0.78 0.09 0.11 0.150.06 0.07 0.24 0.08 0.0006 0.0006 0.0027 0.009 123 25 Inv. ex. 120 26Inv. ex. 76 27 Inv. ex. 108 28 Inv. ex. 128 29 Inv. ex. 95 30 Inv. ex.142 31 Inv. ex. 1.13 132 32 Inv. ex. 0.10 140 33 Inv. ex. 0.79 0.12 14234 Inv. ex. 1.01 0.07 0.11 80 35 Inv. ex. 0.52 0.01 0.10 73 36 Inv. ex.1.10 0.07 0.06 0.10 0.02 76 37 Inv. ex. 0.61 0.26 0.11 71 38 Inv. ex.0.95 0.09 0.06 0.11 0.62 0.14 101 39 Inv. ex. 0.85 0.11 0.12 0.02 0.250.11 82 40 Inv. ex. 0.94 0.05 0.12 0.13 0.06 0.04 0.36 0.09 0.00050.0006 0.0026 0.010 91

TABLE 3 After induction hardening Tempering at 300° C. Surface at depthfatigue Soft nitriding Induction Hardened of 0.2 mm strength Compoundheating nitrided from Hole (maximum Machineability, layer conditionslayer surface, density hertz no. of Temp. thickness Temp. Time thicknessVicker's (holes/ stress) Ex. Class holes (° C.) (μm) (° C.) (s) (mm)hardness mm²) (MPa)  1 Inv. ex. >1000 552 28 897 1.6 0.40 691 7435 3700 1a Inv. ex. >1000 575 24 899 4.9 0.40 720 8352 3700  1b Inv. ex. >1000575 26 898 4.9 0.41 709 7257 3700  2 Inv. ex. >1000 582 30 877 2.5 0.42705 7432 3700  3 Inv. ex. >1000 566 23 837 4.2 0.42 718 6614 3700  4Inv. ex. >1000 575 21 894 2.9 0.41 691 5457 3700  5 Inv. ex. >1000 58324 881 4.9 0.40 764 6684 3700  6 Inv. ex. >1000 577 23 832 4.4 0.41 8196357 3700  7 Inv. ex. >1000 590 19 854 4.9 0.40 675 8532 3700  8 Inv.ex. >1000 553 21 893 2.4 0.42 894 8533 3700  9 Inv. ex. >1000 598 27 8980.9 0.41 689 7607 3750 10 Inv. ex. >1000 552 20 861 0.08 0.42 709 83813750 11 Inv. ex. >1000 587 25 859 3.2 0.42 757 7006 3800 12 Inv.ex. >1000 588 22 801 2.4 0.40 759 8725 3750 13 Inv. ex. >1000 575 20 8901.8 0.42 679 5469 3750 14 Inv. ex. >1000 582 26 882 0.9 0.41 728 54533750 15 Inv. ex. >1000 591 28 892 0.5 0.40 704 5737 3750 16 Inv.ex. >1000 595 29 855 3.4 0.42 698 5133 3750 17 Inv. ex. >1000 570 22 8852.2 0.42 746 8739 3800 18 Inv. ex. >1000 569 25 893 4.5 0.40 765 81533800 19 Inv. ex. >1000 590 24 882 4.4 0.41 805 7211 3750 20 Inv.ex. >1000 571 22 881 2.5 0.41 801 7724 3800 21 Inv. ex. >1000 580 31 8724.5 0.42 693 7031 3750 22 Inv. ex. >1000 574 27 898 0.5 0.40 909 77593750 23 Inv. ex. >1000 571 24 895 0.8 0.41 715 7058 3750 24 Inv.ex. >1000 588 28 822 3.2 0.41 840 8906 3800 25 Inv. ex. >1000 589 25 8821.2 0.40 831 20880 3800 26 Inv. ex. >1000 582 28 890 2.9 0.42 791 268623800 27 Inv. ex. >1000 593 27 899 2.4 0.41 909 19154 3800 28 Inv.ex. >1000 591 26 896 1.9 0.42 701 42901 3800 29 Inv. ex. >1000 581 25894 2.4 0.42 690 21158 3800 30 Inv. ex. >1000 598 28 882 2.8 0.42 69643255 3800 31 Inv. ex. >1000 583 33 893 1.2 0.41 994 11438 3800 32 Inv.ex. >1001 599 30 888 3.4 0.41 707 20960 3800 33 Inv. ex. >1002 587 29884 1.2 0.42 744 17667 3800 34 Inv. ex. >1000 585 24 889 3.4 0.42 79837155 3850 35 Inv. ex. >1000 596 26 893 1.3 0.41 678 29928 3850 36 Inv.ex. >1000 596 29 888 1.3 0.41 841 47635 3850 37 Inv. ex. >1000 581 23882 3.1 0.42 809 34410 3800 38 Inv. ex. >1000 588 28 893 2.8 0.42 77830449 3850 39 Inv. ex. >1000 581 26 898 1.2 0.41 739 10752 3850 40 Inv.ex. >1000 596 27 895 2.1 0.41 790 47202 3850

TABLE 4 Chemical composition (mass %) Ex. Class C Si Mn P S Al O N B 41Comp. ex. 0.15 0.83 1.45 0.022 0.016 0.015 0.002 0.0090 42 Comp. ex.0.55 0.10 0.89 0.009 0.011 0.025 0.001 0.0052 0.0152 43 Comp. ex. 0.450.25 0.50 0.010 0.006 0.450 0.001 0.0160 0.0030 44 Comp. ex. 0.44 1.330.51 0.015 0.051 0.074 0.002 0.0090 48 Comp. ex. 0.55 0.28 0.31 0.0100.025 0.020 0.005 0.0047 49 Comp. ex. 0.56 0.26 0.85 0.005 0.013 0.0300.001 0.0056 50 Comp. ex. 0.11 0.80 2.59 0.013 0.012 0.029 0.001 0.01100.0005 51 Comp. ex. 0.35 0.32 1.44 0.016 0.010 0.025 0.002 0.0053 0.000852 Comp. ex. 0.40 0.25 1.35 0.018 0.011 0.017 0.002 0.0052 0.0009 53Comp. ex. 0.44 0.68 1.23 0.012 0.013 0.013 0.003 0.0047 0.0008 54 Comp.ex. 0.45 0.10 1.11 0.014 0.012 0.025 0.003 0.0051 0.0006 55 Comp. ex.0.47 0.45 1.45 0.013 0.011 0.026 0.004 0.0047 0.0006 56 Comp. ex. 0.550.45 1.34 0.021 0.018 0.052 0.003 0.0050 0.0005 57 Comp. ex. 0.44 0.860.30 0.022 0.024 0.027 0.002 0.0047 0.0006 58 Comp. ex. 0.45 0.25 0.250.005 0.015 0.030 0.001 0.0050 0.0010 59 Comp. ex. 0.55 0.25 0.33 0.0050.013 0.029 0.001 0.0047 0.0006 60 Comp. ex. 0.55 0.25 0.28 0.003 0.0100.033 0.001 0.0053 61 Comp. ex. 0.11 0.12 0.79 0.009 0.010 0.020 0.0010.0090 62 Comp. ex. 0.20 0.25 0.25 0.010 0.017 0.015 0.001 0.0047

TABLE 5 (Continuation of Table 4) Chemical composition (mass %) Ex.Class Cr Mo W V Nb Ti Ni Cu Ca Mg Zr Te Mn/S 41 Comp. ex. 91 42 Comp.ex. 81 43 Comp. ex. 83 44 Comp. ex. 0.51 1.43 10 48 Comp. ex. 0.60 0.110.37 12 49 Comp. ex. 0.49 0.59 0.51 0.0018 65 50 Comp. ex. 0.050 216 51Comp. ex. 144 52 Comp. ex. 1.89 0.41 123 53 Comp. ex. 0.91 95 54 Comp.ex. 0.48 0.07 0.31 93 55 Comp. ex. 1.34 0.22 0.0020 132 56 Comp. ex.0.15 105 57 Comp. ex. 0.54 0.09 0.30 0.10 13 58 Comp. ex. 0.50 0.0016 1759 Comp. ex. 0.49 0.0017 25 60 Comp. ex. 28 61 Comp. ex. 0.30 79 62Comp. ex. 0.90 0.50 0.50 15

TABLE 6 After induction hardening Tempering at 300° C. Surface Soft atdepth fatigue nitriding Induction Hardened of 0.2 mm strength Compoundheating nitrided from Hole (maximum Machineability, layer conditionslayer surface, density hertz no. of Temp. thickness Temp. Time thicknessVicker's (holes/ stress) Ex. Class holes (° C.) (μm) (° C.) (s) (mm)hardness mm²) (MPa) 41 Comp. ex. 81 593 15 876 1.4 0.33 595 3312 2900 42Comp. ex. — — — — — — — — — 43 Comp. ex. — — — — — — — — — 44 Comp. ex.189 590 2 850 3.2 0.13 465 4934 2600 48 Comp. ex. 65 559 4 884 5.0 0.29570 2754 2800 49 Comp. ex. 66 585 1 879 3.2 0.09 467 2038 2300 50 Comp.ex. 45 587 12 880 4.7 0.65 575 4271 2900 51 Comp. ex. >1000 590 24 9804.0 0.65 506 2394 2700 52 Comp. ex. >1000 595 25 700 3.5 0.20 398 28162150 53 Comp. ex. >1000 577 20 890 8.0 0.70 540 1865 2900 54 Comp.ex. >1000 580 21 845 0.03 0.07 425 880 2600 55 Comp. ex. >1000 680 5 8374.9 0.10 401 3082 2600 56 Comp. ex. >1000 490 4 880 3.0 0.16 551 11552700 57 Comp. ex. >1000 700 3 886 2.1 0.15 387 4670 2500 58 Comp.ex. >1000 570 2 910 2.5 0.14 507 575 2600 59 Comp. ex. >1000 570 3 6901.2 0.10 401 4366 2200 60 Comp. ex. 33 570 3 920 4.2 0.12 565 2865 270061 Comp. ex. 89 590 10 1000 4.0 0.32 572 3482 2800 62 Comp. ex. 45 650 41000 2.0 0.23 518 1293 2600

TABLE 7 Machining conditions Drill Others Machining speed: Size: φ5 mm ×Opening depth: 90 mm 65 m/min length 168 mm Judgment of life: TotalFeed: 0.17 mm/rev Material: Ceramic number of holes until Lubrication:Mist coating drill breaks (cut off at lubrication (MQL) Cemented carbide1000) Projecting amount: 105 mm

Note that, what is explained above only illustrates embodiments of thepresent invention. The present invention can be changed in various wayswithin the scope of the language of the claims.

INDUSTRIAL APPLICABILITY

As explained above, by soft nitriding, then induction hardening, thesteel for surface hardening for machine structural use of the presentinvention is remarkably increased in hardness at the surface of thesteel material and is increased in softening resistance to thereby givea high surface fatigue strength. The present invention is high in valueof use in industry.

Further, the parts for machine structural use of the present inventioncan be used for power transmission parts of automobiles etc. for which ahigh surface fatigue strength is demanded not only at ordinarytemperature of course, but also under usage conditions of resulting in ahigh temperature of around 300° C., for example, gears, continuouslyvariable transmissions, bearings, constant velocity joints, hubs, etc.They greatly contribute to the higher output and lower cost ofautomobiles etc. and have remarkable advantageous effects in industry.

REFERENCE SIGNS LIST

-   1: cross-sectional hardness distribution of steel material as soft    nitrided-   2: cross-sectional hardness distribution of steel material which is    soft nitrided, then induction hardened-   10: hardened nitrided layer-   20: hole-   30: porous layer-   40: surfacemost layer

The invention claimed is:
 1. A steel part for machine structural use,said steel part being nitrided, then induction hardened at less than900° C. and tempered at 300° C., wherein the steel part has a steelcomposition that comprises, in mass %, C: 0.3 to 0.6%, Si: 0.02 to 2.0%,Mn: 0.35 to less than 1.5%, and Al: 0.01 to 0.5%, is restricted to B:less than 0.0002%, S: 0.0001 to 0.021%, N: 0.003 to 0.0055%, P: 0.0001to 0.03%, and O: 0.0001 to 0.0050%, Fe: balance and unavoidableimpurities, wherein a ratio Mn/S is 70 to 30,000; and, said steel parthas a core of non-nitrided steel and a nitrided layer on the steel partsurface, wherein said nitrided layer has a thickness of 0.2 mm to 0.42mm, a martensite microstructure, a Vicker's hardness of 650 to 994, andholes with 0.1 to 1 μm in an equivalent circle diameter on a scale of5,133 to 47,635/mm²; and, wherein said core has a ferrite-pearlitemicrostructure.
 2. The steel part for machine structural use as claimedin claim 1, wherein the hardened nitrided layer has a thickness of 0.4mm or more.
 3. The steel part for machine structural use as claimed inclaim 1, wherein the steel part is nitrided at a temperature in therange of 500 to 600° C.